Introduction

Availability is the annual percentage rate that a system is functional and working. Azure offers SLA-graded availability solutions for Virtual Machines by hosting them in multiple data centers around the world. This article provides you with an overview of
the availability features of Azure.

Availability Set

A group with two or more virtual machines in the same Data Center is called Availability Set, this ensures that at least one of the virtual machines hosted on Azure will be available if something happens. This configuration offers 99.95% SLA.

Note

Behind the scenes an availability set is a kind of clustering with not sync between VMs, it just try to keep the VMs up and running.

At the image below we can see a basic scenario of two Web Servers and two Database Servers.

Configure an Availability Set

We can configure an Availability Set ONLY when we deploy a New Virtual Machine,
we can't add an existing Virtual Machine to an Availability Set.
At the image below we can see the Availability Set option on Azure Portal.

Availability Zones

This is the next level of Azure Virtual Machines high-availability, because Virtual Machines are in different physical locations within an Azure Region. It can be deployed using one or more Virtual Machines in an Azure Region. Availability zones offer 99.99%
SLA.

Configure an Availability Zone

Configuring an Availability Zone is very easy, especially from the Azure Portal. Select the Availability Zone for the Virtual Machines using the Availability Zone drop-down. Your managed disk and, if you have one, your public IP will be deployed into the same
zone. Note that for 99.99% SLA, you must deploy at least two virtual machines into at least two different zones.

Differences between Availability Set and Availability Zone

At the table below we can see the two main differences between Availability Set, Zone

Availability Set

Availability Zone

Protect from Hardware failures within data centers

Protect from entire data center failure

SLA 99.95 %

SLA 99.99%

Azure VM SLA

Below are the SLA's for the Azure Virtual Machines.

99.9%, Single VM

99.95%, 2 or more VMs in an Availability Set

99.99%, 2 or more VMs into Availability Zone

Note

This Service Level Agreement for Microsoft Online Services (this “SLA”) is a part of your Microsoft volume licensing agreement (the “Agreement”). If Microsoft does not achieve and maintain the Service
Levels for each Service as described in this SLA, then you may be eligible for a credit towards a portion of your monthly service fees.

Conclusion

Microsoft Azure surprises us day by day. In this post we talked for two of the options offered to users for Availability. These are Availability Sets and Availability Zones, which are very easy to be deployed in both ways, using Azure Portal and Powershell.